Skilled practitioners in the field of thermoplastic resins recognize that the wide range of applications for resins requires differing types of behavior in burning or flame contact conditions. It is also well understood that any application requires cost/benefit optimization of a resin. For example, for the carbonate polymers used in preparing extruded sheet for glazing or other applications it is desired to achieve the Underwriters Laboratory (UL-94) V-2 rating, preferably to achieve this rating simultaneously at thicknesses in the range of one eighth and one sixteenth of an inch (0.125 and 0.0625 inch), while retaining the additives under melt conditions and maintaining the necessary levels of thermal stability, toughness, processability, hydrolytic and chemical resistance, appearance and optical properties.
As is recognized, V-2 performance of the resin characterizes a certain behavior of an article or part molded from the resin under burning conditions. The V-2 rating requires the polymer resin part to self-extinguish within 30 seconds or less after application of a flame from a Bunsen burner that initially ignites the resin part. For the UL-94 test, the resin to be tested is molded into bars having a standard size and shape and 5 bars are tested. In a controlled environment protected from drafts and under specified temperature and humidity conditions, the bars are held in a vertical position while a blue 0.75 inch (20 millimeter) flame from a Bunsen burner is applied to the center of the lower end of the part for ten seconds. The center of the flame is applied to the middle point of the bottom edge of the bar so that the top of the burner is 10 millimeters below the lower end of the specimen. Typically, for a V-2 rated resin, the part initially ignites and is then timed until it self extinguishes (T-1). As soon as the burning part extinguishes, the flame is reapplied for a second ten second time period. Again, the time is recorded until the part self extinguishes (T-2). If the part does not ignite at the first and/or second flame application T-1 and/or T-2 are zero seconds. This is repeated for all 5 bars. If each of the 5 bars self-extinguishes within 30 seconds after each application of the flame and the total burn times (T-1 plus T-2) for all five bars is less than 250 seconds, the resin is rated at least V-2. If any of the 5 bars fails to self-extinguish within 30 seconds after either flame application (T-1 or T-2 are greater than 30 seconds), the resin fails the V-2 test.
Although requiring the polymer part to stop burning within 30 seconds, a V-2 rating in this UL test does acknowledge that molten, flaming polymer drips off the test part under burning conditions and ignites cotton placed underneath. If there are no polymer drips that ignite the cotton, the resin is classified as V-1. The higher UL-94 ratings (V-0 or 5V) are based on shorter flame extinguish times or elimination of the dripping phenomena.
It should be noted that the dripping often removes the ignited polymer from the part and may actually be responsible for stopping the burning of the part. It should also be noted that the user of such resins has to determine the suitability of these resins for any particular application taking into account the dripping polymer, which may be flaming or burning. Any use herein of terms such as ignition resistant or flame retardant, or of UL-94 ratings or other numerical values derived from tests related to burning or the application of a flame, are not intended to reflect hazards presented by the disclosed carbonate polymer compositions or any other material under actual fire conditions. As used herein, the terms "ignition resistance" or "ignition resistance additives" may include or induce the situation where the molten, flaming polymer portion drips off the test part under burning conditions and the test part thereby stop burning.
As used herein to rate and categorize the polymer compositions as to their flame contact behavior, the UL-94 evaluations of the polymer compositions are based only on the performance as a natural, unpigmented resin. When the Underwriters Laboratory performs the test and issues an official UL rating, pigmented and natural versions of the resins are evaluated and all must meet the requirements for a particular rating in order for that polymer composition to receive the rating.
In the past, most polycarbonate resins having melt flow rates in the range of 20 grams per ten minutes and lower had inherently achieved the Underwriters Laboratory (UL-94) V-2 rating at thickness down to one eighth and one sixteenth of an inch without the incorporation of further additives. This was due to their relatively low tendency to burn (compared to many other polymers) and their tendency to reduce in melt strength and drip under burning conditions, extinguishing the burning of the part by removing the burning polymer from the part being tested. It appears that improvements or changes in the polymer process or quality have increased the melt strength of the carbonate polymers to the point that the polymer does not drip as readily. Although the higher melt flow rate (lower molecular weight) carbonate polymers not containing any further additives still have a V-2 rating in the UL-94 test, it has now been found that carbonate polymers having melt flow rates in the range of 20 grams per ten minutes and lower that would previously have been V-2 rated without further ignition suppressing additives, do not drip until after the polymer has burned for an extended time and the desired V-2 performance is not obtained.
In attempting to now modify the behavior of a polymer under burning or flame contact conditions, a skilled practitioner in this field recognizes there are countless additives and additive combinations that are taught to provide ignition resistance and/or affect the melt strength or dripping behavior of carbonate polymers. Since V-2 performance was already practically inherent in many unmodified carbonate polymers, most efforts had been directed to developing additive formulations for achieving the higher V-1, V-0 or 5V UL ratings based on shorter flame extinguish times or elimination of the dripping phenomena. Although levels of the additives were always desired to be minimized to reduce the accompanying loss of the carbonate polymer properties, there is no teaching as to which additives and/or which levels were able to increase the tendency of the polymer to drip under flame contact conditions, thereby providing carbonate polymers having a V-2 UL-94 rating and simultaneously maintaining the otherwise good balance of the unmodified carbonate polymer physical and optical properties.
Among the large number of potential additives for carbonate polymers generally, a variety of phosphorous compounds are known for use as additives in carbonate polymer compositions to achieve incombustibility or ignition resistance of the composition. Phosphorous esters such as the tri(phenyl) phosphates described in U.S. Pat. No. 5,258,432, and the oligomeric phosphates described in U.S. Pat. No. 5,204,394, are representative of some of the phosphorous compounds which have been disclosed as flame retardant additives with or without other additives. In U.S. Pat. No. 5,109,044 haloaryl phosphates are used in ignition resistance formulations in carbonate polymer blends. In JP 59-45,351-A (1984) it is taught that levels of at least 1 weight percent ("wt %") aromatic polyphosphate additive provides incombustible polyamide or polycarbonate compositions. In JP 59-202,240-A (1984) it is also taught that levels of at least 5 weight percent polyphosphoric acid esters provide flame retardance in a range of polymer compositions.
U.S. Pat. No. 3,852,393 discloses a flame retardant blend of polycarbonates with ABS resins and a halogenated hydrocarbon flame retardant. U.S. Pat. No. 4,463,130 discloses a flame retardant blend of polycarbonates with a halogenated flame retardant, polystyrene, polytetrafluoroethylene resins, and a polyphosphate. U.S. Pat. No. 4,563,496 discloses the use of a combination of halogenated compounds and metal oxides to improve the ignition resistance of blend of polycarbonate and acrylonitrile-butadiene-styrene copolymer (ABS). U.S. Pat. No. 4,692,488 discloses a blend of a polycarbonate, a copolymer of styrene and acrylonitrile, triphenylphosphate, and polytetrafluoroethylene.
The article by Joseph Green "PB-460 and PB-528 Brominated Phosphate Ester Flame Retardants" (Fire Retardant Chemicals Association Meeting, Grenelefe, Fla., Mar. 21, 1988) discloses that PB-460 and PB-528 are brominated aromatic phosphate esters with 60-70 percent bromine and 3-4 percent phosphorus. The article by Joseph Green "Brominated Phosphate Ester Flame Retardants for Engineering Thermoplastics" (Fire Retardant Chemicals Association Meeting, San Antonio, Tex., Mar. 12, 1989) discloses that PB-460 is a brominated aromatic phosphate ester with 60 percent bromine and 4 percent phosphorus and that when blended at high concentrations (17.5 weight percent) with Teflon 6C, into blends of polycarbonate and ABS resins, the blend has a V-0 UL-94 rating.
Alkali metal sulfonamide (also referred to as sulfimide) compounds are disclosed for use in carbonate polymer formulations in U.S. Pat. Nos. 4,254,015; 4,486,560; 4,532,284; 4,626,563; 4,727,101; 4,837,258; 4,929,663 and 5,276,077. Other alkali metal sulfonate salts are also suggested for use in carbonate polymers in U.S. Pat. Nos. 4,092,291 and 4,214,062.
Although the references disclose the incorporation of various phosphorous-containing additives into a carbonate polymer along with a very large number of other types of additives, there is no teaching as to which additives and/or which levels were able to provide carbonate polymers having a V-2 UL-94 rating, preferably both at a thickness of one eighth of an inch (1/8") and a thickness of one sixteenth of an inch (1/16"), and maintain or improve the other desirable properties of the resin. It is consequently an object of this invention to obtain a carbonate polymer having the desired V-2 performance level as well as maintaining the necessary levels of thermal stability, toughness, processability, hydrolytic and chemical resistance, appearance, optical properties and retention of the additives under melt processing conditions.